Two stage processing distributed fiber optic sensing (dfos) interrogator for acoustic modulated signals

ABSTRACT

Distributed fiber optic sensing systems (DFOS) methods, and structures that employ DVS/DAS point sensors and a two-stage processing methodology/structure that advantageously enable point sensors to send sensor data at any time—thereby providing significant processing advantages over the prior art.

CROSS REFERENCE

This disclosure claims the benefit of U.S. Provisional PatentApplication Ser. No. 63/006,806 filed 8 Apr. 2020 the entire contents ofwhich is incorporated by reference as if set forth at length herein.

TECHNICAL FIELD

This disclosure relates generally to distributed fiber optic sensing(DFOS) systems, methods, and structures. More particularly, it pertainsto a two stage processing DFOS interrogator for acoustic modulatedsignals.

BACKGROUND

Distributed fiber optic sensing systems, methods, and structures thatemploy distributed vibration sensing (DVS)/distributed acoustic sensing(DAS) point sensors have shown great promise and numerous benefits in avariety of contemporary applications. Such systems, however, generallyrequire that the point sensor(s) send sensor data at a fixed interval orrequire an interrogator to continuously collect and process data fromeach point sensor employed in the system.

SUMMARY

The above problem is solved and an advance in the art is made accordingto aspects of the present disclosure directed to distributed fiber opticsensing systems (DFOS) methods, and structures that employ DVS/DAS pointsensors. In sharp contrast to the prior art, systems, methods, andstructures according to aspects of the present disclosure advantageouslyenable such point sensors to send sensor data at any time—therebyproviding significant processing advantages over the prior art.

As such, systems, methods, and structures according to the presentdisclosure represent a significant advance in the art inasmuch as theinterrogator is able to detect “abstracted” vibration data/signals inaddition to any original (per-sample) signal(s) from point sensors—eachlocated at a different point along the fiber. As used herein, “Abstract”refers to the processed result from the original signal, which mayinclude low and/or high pass filters, power accumulation, etc., tosuppress noise while strengthen the active signal. This result isconsidered as a “vibration”, which has much lower rate than the originalsignal, to reduce software processing complexity while providing all thenecessary information.

In one illustrative embodiment, systems, methods, and structuresaccording to aspects of the present disclosure employ a two stageoperation in the transmitter—which may be part of the point sensor—tosend data, namely 1) an alarm stage and 2) a data sending stage.

In the alarm stage, the transmitter sends a short signal that triggersthe interrogator to switch from a vibration detection mode into aper-sample detection and demodulation mode. In the data sending stage,the transmitter modulates sensory data into vibration/acoustic signal.Correspondingly, for each point sensor, the interrogator will includetwo states, a vibration detection state, and a demodulation state.

It operates in the vibration detection state when there is no activityby a point sensor, which is generally equivalent to processing otherfiber locations that do not have associated point sensors. Once avibration is detected (which is from a point sensor's “alarm” signal),it switches operation to demodulation stage, processes the signalsample-by-sample, and recovers original data. Between the alarm stageand the data sending stage, the transmitter waits a pre-set. fixedinterval of time, which is the interrogator's maximum switching latency.The alarm signal uses a pattern (such as a specific frequency, or apattern of sequence) that is believed most effective for vibrationdetection, while the data signal is tuned—in consideration of thetrade-off between modulation efficiency and correctness of data asreceived at the receiver.

In one illustrative embodiment according to aspects of the presentdisclosure, the transmitter has no alarm stage, and it can send datasignals whenever needed. For each point sensor, the interrogator has aloop buffer that continuously receives data. The data received from eachpoint sensor is handled in two process paths: a vibration detection pathand a buffering path. The vibration detection path follows the sameprocedure as other locations. At the same time, it is written into acorresponding loop buffer, with earlier received in time samples beingreplaced with newer, later received in time ones. When vibration isdetected from a point sensor, the interrogator extracts data from itsloop buffer and performs demodulation to recover any signals. In aproperty configured system, the buffer is sized large enough tocompensate for any vibration detecting latency.

According to certain aspects of the present disclosure, systems,methods, and structures according to the present disclosure employ“coarse processing” and “fine processing” in the interrogator. As we usethe terms herein, “coarse processing” refers to vibration detection—afeature of a DVS/DAS that is usually performed by dedicated logic suchas FPGA (Field Programmable Gate Array) firmware or an ASIC (ApplicationSpecific Integrated Circuit)—resulting in an “abstracted” output thatexhibits a much smaller data volume than provided by the prior artmethods. As further used herein, “fine processing” is signaldemodulation, using either firmware/ASIC or software—with output datacontaining the detailed information. Demodulation is only performed whenvibration(s) are detected—thereby reducing required processing resourcesand/or processing complexity. As those skilled in the art willunderstand and appreciate by our disclosure, our two level approachallows for a software demodulation, which is advantageously andsurprisingly more cost effective and flexible for future upgrades.

Finally, according to aspects of the present disclosure—from the pointof view of a point sensor—the point sensor may send data wheneverneeded, rather than using dedicated time slot—and not when it is notneeded. In an illustrative embodiment according to an aspect of thepresent disclosure wherein a point sensor is a hazard detection sensor,It actively monitors for hazardous conditions and immediately reportonly upon detection of such hazard, otherwise the data transmissionfunction is disabled—thereby conserving power.

BRIEF DESCRIPTION OF THE DRAWING

A more complete understanding of the present disclosure may be realizedby reference to the accompanying drawing in which:

FIG. 1 is a schematic diagram illustrating a prior art distributed fiberoptic sensor system (DFOS);

FIG. 2 is a schematic diagram illustrating a DFOS system including apoint sensor using modulated acoustic signal to convey information tointerrogator according to aspects of the present disclosure;

FIG. 3 is a schematic diagram illustrating demodulation/decoding from afully recovered distributed acoustic sensing (DAS)/distributed vibrationsensing (DVS) signal according to aspects of the present disclosure;

FIG. 4 is a flow diagram of in illustrative method according to aspectsof the present disclosure;

FIG. 5 is a schematic diagram illustrating a signal outputting sequenceat a point sensor according to aspects of the present disclosure;

FIG. 6 is a flow diagram of in illustrative method according to aspectsof the present disclosure;

FIG. 7 is a schematic block/flow diagram of an illustrative methodaccording to aspects of the present disclosure; and

FIG. 8 is a schematic block/flow diagram of an overall illustrativemethod according to aspects of the present disclosure.

DESCRIPTION

The following merely illustrates the principles of the disclosure. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the disclosure and are includedwithin its spirit and scope.

Furthermore, all examples and conditional language recited herein areintended to be only for pedagogical purposes to aid the reader inunderstanding the principles of the disclosure and the conceptscontributed by the inventor(s) to furthering the art and are to beconstrued as being without limitation to such specifically recitedexamples and conditions.

Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat any block diagrams herein represent conceptual views ofillustrative circuitry embodying the principles of the disclosure.

Unless otherwise explicitly specified herein, the FIGs comprising thedrawing are not drawn to scale.

Previously, we reported a uni-directional signal transmit method usingdistributed fiber sensing in which we disclosed a DOFS system usingDVS/DAS point-sensors that conveyed sensor data to an interrogator viaits attached sensor fiber. As we noted, such an inventive arrangementenabled the collection of point-sensor data from locations tens of milesaway from the interrogator while avoiding use of a communicationnetwork. Subsequently, we disclosed using frequency modulation forunidirectional signal transmission through distributed vibration oracoustic sensing wherein we employed frequency-modulation in thepoint-sensor to encode data sent to the interrogator. As notedpreviously however, such data needed to be sent at fixed intervals—orrequire the interrogator to continuously collecting and processing thedata sent from each point sensor.

Those skilled in the art will understand and appreciate that this methodof sending (from point-sensor's point of view) point sensor data atfixed intervals cannot be used in applications that may—for example—sendan alarm out of a regular time slot. Such method also requires asynchronization scheme to compensate clock skew that arises between thepoint sensor and the interrogator. And since the interrogator mustreceive and process data continuously, there is significant processingoverhead and power required—especially in configurations having a largenumber of individual point sensors.

Advantageously—and in sharp contrast—systems, methods, and structuresaccording to aspects of the present disclosure enable point sensors tosend data to an interrogator at any time—using substantially lessprocessing power as compared to the prior art systems and methods.

By way of some additional background, we note that distributed fiberoptic sensing (DFOS) is an important and widely used technology todetect environmental conditions (such as temperature, vibration, stretchlevel etc.) anywhere along an optical fiber cable that in turn isconnected to an interrogator. As is known, contemporary interrogatorsare systems that generate an input signal to the fiber anddetects/analyzes the reflected/scattered and subsequently receivedsignal(s). The signals are analyzed, and an output is generated which isindicative of the environmental conditions encountered along the lengthof the fiber. The signal(s) so received may result from reflections inthe fiber, such as Raman backscattering, Rayleigh backscattering, andBrillion backscattering. It can also be a signal of forward directionthat uses the speed difference of multiple modes. Without losinggenerality, the following description assumes reflected signal thoughthe same approaches can be applied to forwarded signal as well.

As will be appreciated, a contemporary DFOS system includes aninterrogator that periodically generates optical pulses (or any codedsignal) and injects them into an optical fiber. The injected opticalpulse signal is conveyed along the optical fiber.

At locations along the length of the fiber, a small portion of signal isreflected and conveyed back to the interrogator. The reflected signalcarries information the interrogator uses to detect, such as a powerlevel change that indicates—for example—a mechanical vibration.

The reflected signal is converted to electrical domain and processedinside the interrogator. Based on the pulse injection time and the timesignal is detected, the interrogator determines at which location alongthe fiber the signal is coming from, thus able to sense the activity ofeach location along the fiber.

Those skilled in the art will understand and appreciate that byimplementing a signal coding on the interrogation signal enables thesending of more optical power into the fiber which can advantageouslyimprove signal-to-noise ration (SNR) of Rayleigh-scattering based system(e.g. distributed acoustic sensing or DAS) and Brillouin-scatteringbased system (e.g. Brillouin optical time domain reflectometry orBOTDR).

FIG. 1 is a schematic diagram illustrating a prior art distributed fiberoptic sensor system (DFOS). As may be observed from this figure, theinterrogator includes an optical signal source that may further includea laser, electrical signal generator, a modulator, amplifier(s); acirculator that directs source signals to a sensing fiber and returned,backscattered light to an optical detector; a fiber link conveysbackscattered light including environmental information; an opticaldetector that may include photo diode, amplifier(s), to convertreflected optical signal to electrical domain; analog-to-digitalconverter(s) (ADC) and digital signal processor(s) (DSP) to convert adetected analog signal to the digital domain and perform digital signalprocessing to decode sensed data and resulting information.

As those skilled in the art will appreciate, the source interrogationsignal can be an optical pulse (non-coded sensing) or a desired codesequence (for coded case), wherein each location along the length of thefiber reflects a small portion of the optical interrogation signal. Thereflected signal carries information that the interrogator senses. Incontemporary systems, an optical detector may employ—for example—adirect detection or coherent detection scheme. Note that the optical LOis an optical signal produced by a local laser source for coherentdetection, and a synchronization signal from the signal source is usedto indicate a starting location.

Operationally, signals resulting from Rayleigh backscattering exhibits aphase change that is linear to a vibration or stretch level experiencedby the sensing optical fiber. A coherent detector followed bycorresponding signal processing is able to extract the phase change,thus able to fully recover the vibration or stretch. Such operation isusually used to detect a vibration of an acoustic frequency. Note thatan upper limit of the detecting frequency is determined by the opticalpulse or code repetition rate—which in turn is limited by optical fiberlength. For example, a 5 km fiber limits the repetition rate to notexceed 20 kHz, which results in a 10 kHz upper limit of detectableacoustic frequency. We note that a Rayleigh backscattered signal alsoexhibits an amplitude change caused by a vibration/disturbance to theoptical fiber, though the response may not be linear. A direct detectionoptical receiver can be used to detect the amplitude change andtherefore may be used in a system to monitor the vibration, which isknown in the art as distributed vibration sensing (DVS).

As noted previously, we previously reported an approach in which one ormore point sensors were located along a length of sensingfiber—generally where needed according to application. Such sensorsgenerated an acoustic frequency signal which carried sensory informationback to the interrogator. The interrogator subsequently extracted theacoustic signal, decoded the signal to recover an original message, andassociated that message with the specific point sensor along the fiber.

Advantageously, and as will be readily appreciated by those skilled inthe art, point sensors employed can be any of a variety of types, forexample a weather station, a gas sensor, or even a camera—orcombinations thereof. In one embodiment, the point sensor includes amodulator which modulates sensed data or results into an acousticfrequency within the fiber sensing range. The modulator's output drivesa vibration generating device which converts an electrical signal intomechanical vibrations, such as a speaker or other components, that inturn stimulates the optical fiber. This is illustratively shown in FIG.2 which is a schematic diagram illustrating a DFOS system including apoint sensor using modulated acoustic signal to convey information tointerrogator according to aspects of the present disclosure.

Operationally, the interrogator uses a substantially same pre-processingprocedure as DAS/DVS processing, and extracts the acoustic/vibrationsignal (linear or non-linear) of pulse or code repetition frequency. Thesignal is further processed for application-specific purpose, such asvibration detection. In addition, for signals from locations of thepoint sensors, they are also passed to a demodulator for data recovery.This is illustratively shown in FIG. 3 which is a schematic diagramillustrating demodulation/decoding from a fully recovered distributedacoustic sensing (DAS)/distributed vibration sensing (DVS) signalaccording to aspects of the present disclosure.

With this extended discussion in place, we now note that systems,methods, and structures according to aspects of the present disclosureadvantageously employ two levels of processing inside the interrogator:coarse level vibration detection, and fine level demodulation.

Coarse processing uses the vibration detection feature, which typicallyhas the procedure of filtering to suppress noise and/or DC elements, anda power accumulator to enhance the signal while reduce the output rate.Fine level demodulation is processing the signal of a locationsample-by-sample, using corresponding demodulation scheme (such asfrequency, phase, amplitude, and so on) to recover the data sent from apoint sensor. Here “sample” refers to one sample for the specificlocation, which has the rate of pulse or code repetition frequency. Finelevel demodulation is only invoked if there is vibration detected fromcoarse processing.

In one example system, vibration is detected using FPGA firmware or anASIC. The output is a level of vibration for a given period, for eachlocation (in terms of the system's spatial resolution). For vibrationoutput from locations of the point sensors, a thresholder is used todetermine whether further demodulation is needed. If the vibration isabove a pre-determined threshold, prior-vibration-detection samples fromthat location are sent to the demodulator for demodulation.Advantageously, such demodulation can be performed in software, orsoftware plus hardware/firmware accelerator, or hardware/firmware only,all executing on a processor. This procedure is illustratively shown inFIG. 4 is a flow diagram of in illustrative method according to aspectsof the present disclosure.

In one illustrative approach according to aspects of the presentdisclosure once there is data to send, a point sensor uses two stages ofoperation such as is illustratively shown in FIG. 5 which is a schematicdiagram illustrating a signal outputting sequence at a point sensoraccording to aspects of the present disclosure. with reference to thatfigure, it may be observed that the first stage is outputting an alarmsignal, to allow the interrogator to get ready for processing, and thesecond stage is for modulated data. Note that an illustrative alarmsignal may use a specific frequency or pattern that produces a maximumresponse at the interrogator's vibration output. The duration can be thefraction or multiple of one vibration result period, depending on thedetection sensitivity and/or required detection confidence. In oneexample, the point sensors closer to the interrogator can have shorteralarm signal than those farther away. In between the alarm stage and themodulated data stage is the waiting period, which is set to theinterrogator's maximum latency from the end of the alarm signal to thestate ready for demodulation.

With the above signal sequence, once the interrogator detects the alarmfrom a point sensor (i.e., the vibration output from that location ishigher than pre-set threshold), it passes pre-processor output signalfrom that location to the demodulator, to get the point sensor's data.

In yet another approach according to aspects of the present disclosure,a point sensor may send data whenever needed. In operation, a vibrationis detected through a data signal. The interrogator uses the same signalfor both vibration detection and demodulation. Operationally, theinterrogator employs a loop buffer for each point sensor. While data isinput to the vibration detector, data from the point sensor(s) are alsowritten into a corresponding loop buffer. The buffer size for each pointsensor is able to compensate the vibration detection and demodulationlatency. Once vibration from a point sensor is detected, the samples inits loop buffer are read and used for demodulation. Subsequent samplescan be written into the loop buffer then read, or directly input intothe demodulator. The block diagram of such operation is illustrativelyshown in is shown in FIG. 6, which is a flow diagram of in illustrativemethod according to aspects of the present disclosure.

Advantageously, the loop buffer can be part of dedicated hardware, forexample pre-allocated from on-chip memory or external memory (such asSDRAM or SRAM) that is managed by FPGA firmware or ASIC, or allocatedand managed by software sharing the software's SDRAM.

FIG. 7 is a schematic block/flow diagram of an illustrative methodaccording to aspects of the present disclosure. In the detailedprocedure as given in FIG. 7, for samples from point sensor p'slocation, (s{circumflex over ( )}p)i, (s{circumflex over ( )}p)i+1,(s{circumflex over ( )}p)i+2, . . . , are inputted into the vibrationdetector, and written into sensor p's loop buffer at the same time.Assume the vibration detector uses/samples from each location as a blockto generate one output. From the vibration processing of sample block{(s{circumflex over ( )}p)i+k, . . . , (s{circumflex over ( )}p)i+k+l},the output value exceeds threshold v, then the demodulator is notifiedto read samples from the loop buffer. There is possibility that thereare several data samples fall into the previous vibration processingblock, yet not reach threshold v (for example, only 2 data samplesversus a 10-sample vibration processing block which is not strongenough), so the demodulator needs to read from those that fall inprevious vibration processing block. To make it simple, the demodulatorreads from sample (s{circumflex over ( )}p)i+k+l}.

The solution includes a DAS or DVS interrogator, its associated sensingfiber, and point sensors that attached to the fiber. The point sensorsuse acoustic or vibration modulation to convert its sensed data tosignal that can be detected by the interrogator, through the opticalsensing signal inside the fiber. In terms of this communication, a pointsensor works as the transmitter, while the interrogator works as thereceiver, so it is unidirectional.

The interrogator uses vibration detection mechanism to monitor whetherthere is activity from the point sensors. The vibration detector mayinclude filter and power accumulator, that outputs activity in coarselevel. The detection output is connected to a thresholder, which tellsthe demodulator whether to demodulate signal from a point sensor or not.If the vibration is above a threshold, the demodulator will take samplesbelonging to that point sensor and run demodulation, which we call finelevel processing.

The transmitter may provide alarm signal before sending valid data. Thealarm signal is selected to have the optimum response at theinterrogator's vibration detector. Upon receiving the alarm signal,i.e., detecting a valid vibration from a point sensor, the interrogatorsets its demodulator to receive samples from that point sensor andstarts demodulation. The transmitter has a waiting period between alarmsignal and valid data, to accommodate the receiver's handling delay.

Alternatively, a transmitter does not have an alarm stage. It sends outdata whenever needed. Inside the interrogator there is a loop buffer foreach transmitter. Each received sample from a point sensor is handled bythe vibration detector and written into the loop buffer at the sametime. When there is vibration detected, the demodulator is notified toread from the loop buffer and run demodulation.

FIG. 8 is a schematic block/flow diagram of an overall illustrativemethod according to aspects of the present disclosure.

At this point, while we have presented this disclosure using somespecific examples, those skilled in the art will recognize that ourteachings are not so limited. Accordingly, this disclosure should onlybe limited by the scope of the claims attached hereto.

1. A coded distributed fiber optic sensing (DFOS) system comprising: alength of optical fiber cable; a DFOS interrogator system in opticalcommunication with the length of optical fiber cable; an intelligentanalyzer configured to analyze DFOS sensing data received by the DFOSinterrogator system; the distributed fiber optic sensing systemCHARACTERIZED BY a code sequence generator that generates a codedinterrogation sequence; and an out of band signal generator thatgenerates an out-of-band signal (OOS), said OOS being combined with thecoded interrogation sequence prior to introduction into the opticalfiber cable; wherein the OOS is not used by the DFOS system as part ofany DFOS sensing data analysis.
 2. The DFOS system of claim 1 FURTHERCHARACTERIZED BY the OOS is filtered out of any DFOS sensing data priorto its analysis.
 3. The DFOS system of claim 2 FURTHER CHARACTERIZED BYthe OOS is synchronized with the coded interrogation sequence such thatthe combination of the two signals is a signal exhibiting a constantamplitude waveform.
 4. The DFOS system of claim 3 FURTHER CHARACTERIZEDIN THAT the combined signals are output to the optical fiber and usingthe constant amplitude characteristic of the signal an equalized gainfor the coded sequence is produced.
 5. The DFOS system of claim 4FURTHER CHARACTERIZED IN THAT the DFOS system is integrated in that theoptical fiber simultaneously conveys non-sensory telecommunicationsoptical signals.
 6. The DFOS system of claim 5 FURTHER CHARACTERIZED INTHAT DFOS interrogation signals and the non-sensory telecommunicationssignals are wavelength-division-multiplexed into the optical fiber. 7.The DFOS system of claim 6 FURTHER CHARACTERIZED IN THAT the constantamplitude characteristic of the interrogation signals improves anynonlinear cross-talk characteristics with the telecommunications signalsdue to reduced cross-phase modulation—as compared with an operationusing interrogation signals that did not exhibit the constant amplitude.